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  Java Practices -> Data access objects
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<div class='page-title'>Data access objects</div>

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<br>Data Access Objects (DAOs) :
<ul>
<li>can be used in a large percentage of applications - anywhere data
storage is required.</li>
<li>hide all details of data storage from the rest of the application.</li>
<li>act as an intermediary between your application and the database. They
move data back and forth between Java objects and database records.
<li>allow ripple effects from possible changes to the persistence mechanism to be confined
to a specific area.</li>
</ul>

<p>For simple DAOs, various implementation styles are possible :
<p><b>Style 1 - instance methods</b>
<p><tt>MessageDAO dao = new MessageDAO();</tt>
<br><tt>List&lt;Message&gt; messages = dao.fetchRecentMessages();</tt>
<p>This is the recommended style. Create an ordinary object in the usual way, and use its services.

<p><b>Style 2 - variation on Style 1</b>
<p><tt>List&lt;Message&gt; messages = new MessageDAO().fetchRecentMessages();</tt>
<p>The DAO is created and used on the same line. This style seems a bit less legible than Style 1.

<p><b>Style 3 - static methods</b>
<p><tt>List&lt;Message&gt; messages = MessageDAO.fetchRecentMessages();</tt>
<p>Probably the least desirable. One must exercise care that such classes are
thread-safe. This issue usually doesn't exist in Styles 1 and 2, if the DAO
has no static members, is short-lived, and there is no possibility of sharing data between threads.

<P><b>Full DAO Design Pattern</b>

<P>It is common to describe Data Access Objects as <a href='http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html'>an extended design pattern</a>. 
That design pattern centers on allowing wholesale changes to a datastore mechanism - for
example, changing from a relational database to using the file system, or some other means of data storage. 
It is only required when multiple storage mechanisms must coexist.

<P><span class='highlight'>It must be stressed that most applications do not need the full DAO design pattern 
as usually described</span>. Most business applications are deployed with
a very particular database in mind. In such cases, allowing a deployer
to configure various types of datastore seems entirely superfluous. If your application
interacts with only one datastore at a time, then the 
<em>full</em> DAO pattern is clearly of dubious benefit. 
Rather, implementing each DAO as a single, ordinary class seems more appropriate and effective.

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<div class='topic-section'>See Also :</div>
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  <a href='TopicActionf6c1-2.html?Id=77'>Data exception wrapping</a> <br>
 
  
  <a href='TopicActione914-2.html?Id=105'>Keep SQL out of code</a> <br>
 
  
  <a href='TopicAction23a3-2.html?Id=123'>Minimize ripple effects</a> <br>
 
  
  <a href='TopicAction8aa5-2.html?Id=128'>Abstract Factory</a> <br>
 
  
  <a href='TopicActiond08d-2.html?Id=187'>Model Objects</a> <br>
 
  
  <a href='TopicAction9e16-2.html?Id=200'>Try pseudo-persistence for mock ups</a> <br>
 
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